From the Contact search bar, enter the name of the contact you would like to add to a particular Contact Group.
From the contact's record, select 'Campaign Info' from the relationship tree on the right-hand side to jump to the campaign section of the contact record.
From the 'Campaign Info' section, select the 'Change Campaign' button in the lower left corner of the campaign details section.
From the returned list of campaigns, select the campaign you wish to place the contact on.
Once you've selected the campaign you wish to place the contact on, you will be directed back to the contact record, and a success message stating that the campaign has been changed and you will be returned in the 'Campaign Info' section.